The general technique of recharging a battery is well known: forcing a current into the battery. However, this technique, while simple, can cause excessive heating of the battery, excessive gassing, and require a prolonged time to fully recharge the battery. The time required to recharge a battery can be reduced by applying a depolarizing (discharging) pulse between charging pulses, as is disclosed in U.S. Pat. No. 3,597,673 to Burkett et al. A further reduction in the charging time and in the heating of the battery can be obtained by waiting for a specified period after the end of the discharge pulse before applying the next charging pulse. This technique is disclosed in U.S. Pat. No. 4,829,225 to Podrazhansky et al. However, it is desirable to further reduce the battery heating and the charging time.
A rechargeable battery, once discharged, requires recharging to restore energy to the battery. Several hours, or more, are typically required to recharge a battery because a conventional battery charger cannot deliver a high charging current without causing overheating of the battery. As is well known, overheating a battery dramatically reduces the life of the battery. Therefore, there is a need for a battery charger which can quickly recharge a battery by providing a high charging current in a manner which does not overheat the battery.
When a battery is charged, internal resistance is created within the battery by the creation of a diffusion layer which results from the migration of positive ions to the negative plate and the migration of negative ions to the positive plate. This diffusion layer, sometimes called a Duffney layer, is not easy to break and creates a higher internal resistance to the flow of ions. When a battery is frozen, this internal resistance is further increased due to the decreased velocity of the ions. This high internal resistance makes it virtually impossible to charge a frozen battery. Therefore, charging a frozen battery by using direct current is extremely difficult and takes an extended period of time. It is therefore desirable to reduce the charging time of a frozen battery.
With specific types of batteries, such as lead acid batteries, the state of the charge can be determined by simply measuring the battery voltage. In particular, the battery voltage will rise until the battery is fully charged, and then the battery voltage will drop. In lead acid batteries the drop is readily detectable. Therefore, charging systems can determine when to terminate charging based on this change in voltage and avoid unnecessary energy consumption and damage to the battery. In some other types of batteries however, the drop is so small it can be easily masked by noise or normal variations in battery voltage. With some battery types, such as NiCad and NiFe, there is not a known indicator which can be utilized to determine the state of the charge. Therefore, conventional battery charging systems are unable to determine the optimum point to terminate charging. Therefore, there is a need to determine the state of the charge for NiCad and NiFe as well as other battery types in order to avoid unnecessary energy consumption and damage to the battery.
A newly constructed battery requires formatting (charging). Depending on the type and size of the battery, this may require 12 hours to several days. The electrolyte is placed in the battery and some electrolyte is absorbed by the plates. The initial chemical reaction generates a great deal of heat and the battery temperature may easily reach 170.degree. F. Once the electrolyte is absorbed by the plates, the temperature will begin to fall, thereby indicating that the absorption (pickling) time is over and the battery is ready for formation. An electrolyte temperature of 135.degree. F. to 145.degree. F. is desirable for battery formation. A high charging current is desired in order to reduce the formation time. However, the charging current should not be greater than that required to maintain the desired battery temperature or overheating of and damage to the battery may result. Therefore, there is a need for a battery charger which provides a charging current which minimizes the formation time without overheating the battery.
There is no known method of determining the state of formation of the battery. This inability to determine the state of formation of the battery makes it difficult to determine the optimum point for terminating formation of the battery. Therefore, in order to assure that a battery has been formatted, a battery is typically charged for a fixed amount of time. However, this generally causes overcharging of the battery, wastes energy, causes gassing due to electrolysis of the water, and prolongs the formatting time. If a short time is used, so that gassing does not occur, the battery may not be completely formatted or charged. Therefore, there is a need to determine the state of formation of the battery in order to insure that the battery is completely formatted, in order to avoid unnecessary energy consumption, and in order to reduce the time for the formation process.